Typical water runoff contains a surprisingly large amount of oil and other contaminants. The resulting contamination of natural receiving water incurs enormous annual costs, both financial and environmental. For example, a government study in one published article showed that storm water sampled from street sites contained an “event mean concentration” of 2.2 mg. of oil per liter of runoff water. Shepp, “Petroleum Hydrocarbon Concentrations Observed in Runoff from Discrete, Urbanized Automotive-Intensive Land Uses,” Watershed'96. If one meter of rain per year falls on a street 10 meters wide, then at that observed mean rate, the annual runoff from each kilometer of street will contain about 275 liters of hydrocarbons.
Various systems have been developed to remove hydrocarbons and other chemical contaminants from runoff water. For example, U.S. Pat. No. 6,080,307, and commonly owned with the present application, discloses a system for recovering trash and oil from water passing into storm drains or the like.
Unfortunately, a report of the National Resources Defense Council (“Testing the Waters 2001,” available from www.nrdc.org) makes it dear that such conventional systems for chemical decontamination and debris removal are inadequate for effective purification of runoff water. Eighty-five percent of beach closings and health advisories occurring in 2000 were the result of high bacteria levels, according to the report, and “polluted runoff and storm water caused or contributed to more than 4,102 closings or advisories.” Clearly there remains a need for further improvements in the purification of runoff water to alleviate continued concerns about public health.